Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

On Thu, Sep 27, 2018 at 06:20:23PM +0200, Jann Horn wrote:

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen [off-list ref] wrote:

quoted

As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
version which can acquire filters is useful. There are at least two reasons
this is preferable, even though it uses ptrace:

1. You can control tasks that aren't cooperating with you
2. You can control tasks whose filters block sendmsg() and socket(); if the
   task installs a filter which blocks these calls, there's no way with
   SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.

v2: fix a bug where listener mode was not unset when an unused fd was not
    available
v3: fix refcounting bug (Oleg)
v4: * change the listener's fd flags to be 0
    * rename GET_LISTENER to NEW_LISTENER (Matthew)
v5: * add capable(CAP_SYS_ADMIN) requirement
v7: * point the new listener at the right filter (Jann)

Signed-off-by: Tycho Andersen <redacted>
CC: Kees Cook <redacted>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <redacted>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <redacted>
CC: Tyler Hicks <redacted>
CC: Akihiro Suda <redacted>

If you address the two nits below, you can add:
Reviewed-by: Jann Horn <jannh@google.com>

Thanks!

quoted

 include/linux/seccomp.h                       |  7 ++
 include/uapi/linux/ptrace.h                   |  2 +
 kernel/ptrace.c                               |  4 ++
 kernel/seccomp.c                              | 31 +++++++++
 tools/testing/selftests/seccomp/seccomp_bpf.c | 68 +++++++++++++++++++
 5 files changed, 112 insertions(+)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 017444b5efed..234c61b37405 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h

@@ -83,6 +83,8 @@ static inline int seccomp_mode(struct seccomp *s)
 #ifdef CONFIG_SECCOMP_FILTER
 extern void put_seccomp_filter(struct task_struct *tsk);
 extern void get_seccomp_filter(struct task_struct *tsk);
+extern long seccomp_new_listener(struct task_struct *task,
+                                unsigned long filter_off);

Nit: Sorry, I only noticed this just now, but this should have return
type int, not long. ptrace_request() returns an int, and an fd is also
normally represented as an int, not a long.

Ugh, I could have sworn I checked this. In particular because the
other seccomp code that's called from ptrace returns a long too :)

I'll fix that for the next version, and send a different patch for the
other two.

quoted

 #else  /* CONFIG_SECCOMP_FILTER */
 static inline void put_seccomp_filter(struct task_struct *tsk)
 {

@@ -92,6 +94,11 @@ static inline void get_seccomp_filter(struct task_struct *tsk)
 {
        return;
 }
+static inline long seccomp_new_listener(struct task_struct *task,
+                                       unsigned long filter_off)
+{
+       return -EINVAL;
+}
 #endif /* CONFIG_SECCOMP_FILTER */

 #if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)

diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index d5a1b8a492b9..e80ecb1bd427 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h

@@ -73,6 +73,8 @@ struct seccomp_metadata {
        __u64 flags;            /* Output: filter's flags */
 };

+#define PTRACE_SECCOMP_NEW_LISTENER    0x420e
+
 /* Read signals from a shared (process wide) queue */
 #define PTRACE_PEEKSIGINFO_SHARED      (1 << 0)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 21fec73d45d4..289960ac181b 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c

@@ -1096,6 +1096,10 @@ int ptrace_request(struct task_struct *child, long request,
                ret = seccomp_get_metadata(child, addr, datavp);
                break;

+       case PTRACE_SECCOMP_NEW_LISTENER:
+               ret = seccomp_new_listener(child, addr);
+               break;
+
        default:
                break;
        }

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 44a31ac8373a..17685803a2af 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c

@@ -1777,4 +1777,35 @@ static struct file *init_listener(struct task_struct *task,

        return ret;
 }
+
+long seccomp_new_listener(struct task_struct *task,
+                         unsigned long filter_off)
+{
+       struct seccomp_filter *filter;
+       struct file *listener;
+       int fd;
+
+       if (!capable(CAP_SYS_ADMIN))
+               return -EACCES;
+
+       filter = get_nth_filter(task, filter_off);
+       if (IS_ERR(filter))
+               return PTR_ERR(filter);
+
+       fd = get_unused_fd_flags(0);

s/0/O_CLOEXEC/ ? If userspace needs a non-cloexec fd, userspace can
easily unset O_CLOEXEC; but the reverse isn't true, because it'd be
racy.

Sure, will do.

Tycho